The present invention relates to an electric double layer capacitor, and more particularly to an electric double layer capacitor having polarizable electrodes in solid state.
FIG. 1 is a schematic perspective view illustrative of a cell lamination structure of a conventional electric double layer capacitor. FIG. 2 is a cross sectional view illustrative of a conventional electric double layer capacitor taken along an A-A' line of FIG. 1. FIG. 3 is a cross sectional view illustrative of a cell structure of a conventional electric double layer capacitor of FIG. 1.
With reference to FIG. 1, the cell lamination structure 8 of the conventional electric double layer capacitor comprises gaskets 2, basic cells 5 adjacent to the gaskets 2, terminal electrodes 6 sandwiching the gaskets 2 and the basic cells 5, pressure plates 7 sandwiching the terminal electrodes 6.
With reference to FIG. 2, the gasket 2 surrounds a collector 3 which further surrounds the polarizable electrode 1. The polarizable electrode 1 comprises an activated carbon in solid state such as an activated carbon/polyacetone material. The activated carbon has many pores for containing an electrolyte solution so that charges are accumulated on interfaces between the electrolyte solution and inner walls of the pores of the activated carbon of the polarizablc electrode 1. The collector 3 is made of a rubber or plastic which contains an electrically conductive carbon. The collector 3 is tightly fitted with the gasket 2. The collector 3 may also be made of a metal foil except when an acidic electrolyte solution is used.
With reference to FIG. 3, the polarizable electrodes 1 in the form of a pair are separated by a porous separator 4. An electrolyte solution 9 is confined in a space defined by an inner wall of the gasket 2 and inner walls of the collectors 3. The electrolyte solution 9 is confined between the polarizable electrodes 1 and the inner wall of the gasket 2 and also confined in a large number of the pores of the activated carbon of each of the polarizable electrodes 1. The electrolyte solution 9 is sealed with the collector 3 and the gasket 2. A withstand voltage of the electric double layer capacitor is limited by an electrolyzing voltage of the electrolyte solution 9, for which reason the necessary number of the basic cells 5 are connected in series to obtain a required withstand voltage to thereby form a laminated structure of the basic cells 5. The pressure plates 7 are provided to give a constant pressure to the laminated structure of the basic cells 5, so as to reduce contact resistances between the basic cells 5 and also between the basic cell 5 and the terminal electrode 6.
In the meantime, the use of the polarizable electrodes 1 results in an increase in capacitance of the electric double layer capacitor. Further, an equivalent series resistance of the electric double layer capacitor could be reduced. In these circumstances, recently new uses of the electric double layer capacitor have become attractive. For example, the electric double layer capacitor may be used in combination with a lead battery to act as a battery for driving starter motors for automobiles. Further, the electric double layer capacitor may be used in combination with a solar battery to act as an auxiliary power supply.
A high reliability is essential commonly to the electric double layer capacitors, but the required characteristics of the electric double layer capacitors are different for the use.
Actually, however, it was difficult for the conventional electric double layer capacitors to keep a high reliability for the following reasons. The conventional electric double layer capacitor has an incomplete sealing structure. If a predetermined voltage is applied to the electric double layer capacitor, then an oxidation-reduction reaction is caused to generate a gas for all kinds of the electrolyte solutions. Particularly when a sulfuric acid solution is used, the amount of the gas generated is maximum even characteristics such as ion conductivity are preferable. As compared to the other electrolyte solutions, the sulfuric acid solution generates a large amount of the gas and largely increases an internal pressure of the cell laminations 8 whereby the cell laminations 8 are expanded for a short time, resulting in a certain possibility of breaking the cell laminations 8.
In order to solve the above problems, it may be effective to use an organic electrolyte solution which is likely to generate a small amount of the gas. However, the use of the organic electrolyte solution is also engaged with a disadvantage in a low ion conductivity which increases the equivalent series resistance. This further causes that ions are enlarged in diameter whereby a movable speed of the ions is delayed. This makes it difficult to use the electric double layer capacitor under a large current condition.
The conventional electric double layer capacitor has an incomplete sealing structure which allows a vaporization of a hydrated electrolyte solution through a gap between the gaskets.
In the above circumstances, it had been required to develop a novel electric double layer capacitor free from the above problems and disadvantages.